Transparent conducting oxide (TCO) thin films have wide applications in optoelectronics. For example, they have been widely used as transparent electrodes in various devices including flat panel displays such as liquid crystal displays (LCDs) and plasma panel displays (PDPs), light emitting diodes (LEDs), solar cells, and thin film transistors. Transparent conductive thin films also have applications as window coatings that confer heat-reflecting, antistatic, and/or defogging properties.
Various TCO materials have been identified and studied in bulk forms and in thin films. They include tin oxide (SnO2) doped with antimony or fluorine, zinc oxide (ZnO) doped with aluminum or gallium, and indium oxide (In2O3) doped with tin. Tin-doped indium oxide (ITO) is the current TCO of choice in most industrial applications, having conductivity of about 2000-4000 S/cm for polycrystalline thin films, a work function of about 4.5 eV, and optical absorption in the blue-green spectral region.
Furthermore, due to recent high demands for flat panel displays, there has been a growing supply deficit for indium. As a result, the price of indium has increased drastically. It was reported that the average price for indium in 2005 was US $900 per kilogram. This presents significant challenges to large-scale introduction of next-generation flat panel display and photovoltaic technologies, as commercial ITO thin films often have indium content near 90 cation %.
Impressive scientific and technological progress has recently been achieved in the area of organic light-emitting diodes (OLEDs), driven by potential applications in a large variety of display technologies. An equal fundamental research motivation has been the desire to better understand and control charge injection into, charge migration through, and radiative recombination in, molecular and macromolecular solids. Over the past few years, increasing activity has focused on improving charge injection efficiency at both OLED cathode/organic and anode/organic interfaces. (See, e.g., J. E. Malinsky, G. E. Jabbour, S. E. Shaheen, J. D. Anderson, A. G. Richter, N. R. Armstrong, B. Kipplelen, P. Dutta, N. Peyghambarian, T. J. Marks, Adv. Mater. 1999, 11, 227). Low work function metals (e.g., Ca, Mg) and combinations with other atmospherically stable metals (e.g., Ag, Al) have been implemented as cathodes, to afford improved luminous quantum efficiencies and lower operating voltages. (C. Zhang, D. Braun, A. J. Heeger, J. Appl. Phys. 1993, 73, 5177; J. Kido, K. Hongawa, K. Okuyama, K. Nagai, Appl. Phys. Lett. 1993, 63, 2627.) In contrast, relatively few materials have been explored as alternatives to Sn-doped In2O3 (ITO) as OLED anodes. As an n-doped, degenerate wide band gap semiconductor, ITO is used in numerous opto-electronics applications (e.g., photovoltaic cells, flat panel liquid crystal displays, “smart” windows, etc.) because of good transmittance in the visible and near-IR, low electrical resistivity, and easy processibility. (H. L. Hartnagel, A. L. Dawar, A. K. Jain, C. Jagadish, Semiconducting Transparent Thin Films, Institute of Physics, Bristol. 1995; Special Issue on Transparent Conducting Oxides, (Eds: D. S. Ginley, C. Bright), MRS Bulletin. August 2000, Vol. 25.)
However, the chemical and electronic properties of ITO are far from optimum for current and future generation OLEDs. Drawbacks include (1) deleterious diffusion of oxygen and In into proximate organic charge transporting/emissive layers (A. R. Schlatmann, D. W. Floet, A. Hillberer, F. Garten, P. J. M. Smulders, T. M. Klapwijk, G. Hadziioannou, Appl. Phys. Lett. 1996, 69, 1764; J. C. Scott, J. H. Kaufman, P. J. Brock, R. Dipietro, J. Salem, J. A. Goitia, J. Appl. Phys. 1996, 79, 2745), (2) imperfect (injection barrier-creating) work function alignment with respect to typical hole transport layer (HTL) HOMO levels (L. Chkoda, C. Heske, M. Sokolowski, E. Umbach, F. Steuber, J. Staudigel, M. Stossel, J. Simmerer, Synthetic Metals 2000, 111, 315; Y. Park, V. Choong, Y. Gao, B. R. Hsieh, C. W. Tang, Appl. Phys. Lett. 1996, 68, 2699; D. J. Milliron, I. G. Hill, C. Shen, A. Kahn, J. Schwartz, J. Appl. Phys. 2000, 87, 572), and (3) poor transparency in the blue region. (J. M. Philips, J. Kwo, G. A. Thomas, S. A. Carter, R. J. Cava, S. Y. Hou, J. J. Krajewski, J. H. Marshall, W. F. Peck, D. H. Rapkine, R. B. V. Dover, Appl. Phys. Lett. 1994, 65, 115.) Several alternative materials have been recently examined as anodes, including TiN, doped Si, Al-doped Zn, and F-doped SnO2. However, all such materials suffer from some combination of poor optical transparency and/or significantly lower work functions than ITO, resulting in poor Fermi level energetic alignment with HTL HOMOs. Efforts continue in the art for an effective alternative to ITO and use thereof in OLED anode and device structures.
Accordingly, there is a desire in the art for low indium content alternative TCO materials that have opto-electrical properties that are superior or comparable to ITO. Preparation techniques that can be used to improve the opto-electrical properties of both existing and new TCO materials also are desired.
Meanwhile, metal-organic chemical vapor deposition (MOCVD) recently has been identified as an attractive growth process for ZITO (zinc-indium-tin-oxide) thin films. To achieve effective growth of thin films by MOCVD, a suitable metal-organic precursor is critical. Ideally, the metal-organic precursor is both highly volatile and thermally stable, and can be easily handled. Most current MOCVD precursors lack at least one of these characteristics.
For example, while zinc is an important component in many new TCO materials, current zinc precursors for MOCVD processes suffer from either poor reproducibility in growth processes or chemical instability. Several zinc compounds, such as liquid diethyl zinc and dimethyl zinc, zinc acetate, and Zn(hfa)2.2H2O.polyether adducts (hfa=1,1,1,5,5,5-hexafluoro-2,4-pentanedionato), have been demonstrated as MOCVD precursors in the growth of zinc-containing oxide thin films. However, diethyl zinc and dimethyl zinc are volatile, pyrophoric liquids which must be handled in an inert atmosphere. They are highly reactive materials and difficult to control in the deposition of multi-component films. In the case of zinc acetate and Zn(hfa)2.2H2O.polyether, the water of hydration must be removed before these precursors can be used effectively. The volatility of zinc acetate also decreases markedly over prolonged deposition runs. Zn(dpm)2 (dpm=2,2,6,6-tetramethyl-3,5-heptanedionato) is another widely-used MOCVD precursor which does not require a co-reactant or pre-treatment. However, it is a solid over a broad temperature range and suffers from sintering at elevated temperatures and during film growth runs. Sintering decreases the surface area of the solid precursor and thereby causes the flux of gaseous zinc species being transported to vary during the film growth process, seriously compromising film compositional control.
Accordingly, there is a desire in the art for improved MOCVD precursors that can be used to prepare TCO thin films.